This invention relates to a compound ceramic material, preferably based on silicon nitride, useful for cutting tools and structural components where the wear resistance and toughness behaviour is important.
Silicon nitride ceramics have many interesting features which have made them extremely suitable for certain wear part applications and as cutting tools. Intrinsic properties such as low thermal expansion, relatively high thermal conductivity and high elastic modulus give the material excellent resistance against thermal shocks and, thus, good toughness behaviour.
However, it is difficult to sinter silicon nitride ceramics to full density without using sintering aids. Such sintering aids include magnesia, alumina, beryllia, zirconia and the rare earth oxides including yttria. Also the related nitrides, oxynitrides or silicates may be used as sintering aids. By applying standard sintering techniques, Si--Al--O--N ceramics may be formed in which Al and O have been substituted from some Si and N atoms in the crystal lattice. The chemical formula for this material may be written Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (where 0&lt;z&lt;4.2). This beta-sialon phase has the same crystal structure as beta--Si.sub.3 N.sub.4.
Another sialon phase, for instance, alpha sialon stabilized by yttria, is M.sub.x (Si,Al).sub.12 (O,N).sub.16, where M=yttrium or other atoms with a suitable atom radius and x is typically between 0 and 2.
By using hot pressing, HIP sintering or similar sintering methods, dense Si--Al--O--N ceramics will be formed in which the silicon nitride is essentially alumina free (or with very low alumina substitution).
The added sintering aids form an intergranular phase during the sintering. The amount is typically 5-20 vol-%. This phase can be amorphous or partly or fully crystalline depending on sintering conditions, composition, etc.
During sintering, the ceramic material is often embedded in a protective powder bed with inert material such as BN or Si.sub.3 N.sub.4 or mixtures of these materials with SiO.sub.2 or other metal oxides. It is important to avoid excessive degradation of the ceramic material by formation of gaseous compounds such as SiO and N.sub.2. The powder provides a protective embedment and a local atmosphere with sufficiently high counterpressure of SiO and N.sub.2. This could be done on the microscale (powder surrounding each compact) or on macroscale (the powder or the powder compacts are used as a gas generator to produce the necessary counterpressure for the whole sintering system). Essentially no solid state reactions take place between the ceramic compacts and the protective powder, only gas reactions.